05/08/2020
READ | "Running Footstrike: What Does Science Tell Us?" by Kevin A. Kirby, DPM
Running Footstrike: What Does Science Tell Us?
From 2010 to 2016, the barefoot running and minimalist running shoe fads came on strong and then suddenly faded away. Even though many of the claims made by the barefoot running and minimalist running shoe zealots have been shown by recent research to be unsupported or patently untrue, some of the strange ideas promoted by these individuals still remain as accepted principles by many within the running community.
One of the most unusual claims still being made by self-proclaimed “running form experts” is that during footstrike, when the foot first hits the ground during running gait, the foot should strike the ground under the center of mass (CoM) of the runner, and not ahead of the runner’s CoM. There are probably a few reasons why this false idea that the foot should strike the ground under the CoM during running is still being preached as the “correct way to run”.
The barefoot/minimalist running shoe advocates often taught that rearfoot- striking running was somehow bad for the runner even though there is no research evidence that rearfoot-striking running causes increased injuries and over 90% of all runners are rearfoot strikers (Kirby KA: Footstrike and Running Form Controversies: What Does the Scientific Evidence Tell Us? Foot Ankle Quarterly, 24(3):109-118, 2013). In addition, teaching a runner to take shorter strides will decrease the impact forces of footstrike, but will also require the runner to take more steps per mile, which may cause increased foot loads over time.
It has been known for over four decades that “over-striding”, taking too long a running stride, is a common problem among beginning runners. My high school cross-country coach from the early 1970s often told newer team members to shorten their stride to improve their running efficiency. Therefore, the idea of reducing running stride length to improve running efficiency is not a new idea generated from the barefoot or minimalist running shoe fads. In fact, scientific research on the biomechanics of running from the early 1980s found that experienced runners will self-select the stride length that optimally decreases the metabolic energy required for running, indicating that experienced runners may be able to achieve their own metabolically-economical and efficient running form (Cavanagh PR, Williams KR: The effect of stride length variation on oxygen uptake during distance running. Med Sci Sports Exer, 14: 30-35, 1982).
The notion that the foot should first strike the ground under the CoM is clearly shown to be false upon close examination of slow motion videos of runners. Slow motion videos of constant-velocity, level running show that the runner’s foot always strikes ahead of their CoM and not under their CoM. The only time a runner would be expected to have their footstrike under their CoM is if the runner was “running in place”, with a forward velocity equal to zero. In other words, if the runner wants to run in a forward direction, then the runner must footstrike with their foot in front of their CoM, not under their CoM.
The arguments that are often made by these self-proclaimed “running form experts” to justify their foot-under-the-CoM-footstrike recommendation is that when a runner foot-strikes with their foot ahead of their CoM, they will develop a braking force on the foot, slowing down the runner. These same self-proclaimed "running form experts" also preach that these posteriorly-directed shear forces that occur when a runner foot-strikes with their foot in front of their CoM are abnormal and are, somehow, a negative influence on running economy. Unfortunately, these claims are far from the truth and have created misconceptions regarding the biomechanics of running.
First of all, it is completely normal for runners in constant-velocity, level running to have their foot strike in front of their CoM. In fact, it is biomechanically impossible to run, without falling, at a jogging or running pace on level ground with the foot striking directly under the runner’s CoM. Secondly, the posteriorly-directed shearing force component of ground reaction force (GRF) seen in force plate analysis of the first half of the support phase is biomechanically required for normal running. [The support phase is the phase in running when the foot is on the ground and is analogous to the stance phase in walking.]
In 1980, Cavanagh and Lafortune published the first major running biomechanics study that analyzed the location, magnitude and direction of the GRF vector resulting from the impact of the foot with the ground during running. They showed that there was a posteriorly-directed GRF vector during the first half of support phase and an anteriorly-directed GRF vector during the second half of the support phase of running. The result of these changes in GRF vector location during running was that there was a posteriorly-directed shearing force from GRF in the first half of support phase and an anteriorly-directed shearing force from GRF in the second half of the support phase of running (Cavanagh PR, Lafortune MA: Ground reaction forces in distance running. J Biomechanics, 13, 397–406, 1980). These findings have been confirmed by multiple other researchers in numerous force plate studies over the past 35 years.
These research findings indicate that the GRF vector tends to always point towards the CoM of the runner throughout the support phase of running. At footstrike, the GRF vector is pointed in a posterior-superior direction, toward the CoM, since the foot is well forward of the CoM at footstrike. At the middle of support phase, the GRF vector is pointing directly superior toward the CoM since the foot is directly underneath the CoM. Finally, at toe-off, at the end of support phase, the GRF vector is pointed in a anterior-superior direction, again toward the CoM, since the foot is behind the CoM at toe-off.
Article reprinted from Kirby KA: "Biomechanics of Running Footstrike - Volume I", December 2016 Precision Intricast Newsletter. In Kirby KA: Foot and Lower Extremity Biomechanics V: Precision Intricast Newsletters, 2014-2018. Precision Intricast, Inc., Payson, AZ, 2018, pp.121-122.
Dr. Kirby's five books may be purchased from Precision Intricast Orthosis Lab at www.precisionintricast.com/shop.
